EP 1548171 describes a drive system for washing machines. The drive system comprises a motor with a large diameter shallow stator and a rotor with magnets external to the stator. The stator is supported on the end of a washing tub as shown in FIG. 2 of that application. The stator has an aperture for a drive shaft to pass through. As shown in FIGS. 2 and 16 of EP patent application 1548171, a rotor, which is to be fixed to the rotating drum of a washing machine, has a ring of permanent magnet material supported on the inside of a steel backing ring. A frame extends between the hub of the rotor (through which the shaft can extend) and the steel backing ring. The backing ring and frame may be formed together. The permanent magnet material is made of a set of curved permanent magnet elements. The permanent magnet material is magnetised after physical construction of the rotor. A typical rotor has more than 30 poles magnetised into the ring of magnetic material. The polarity of the poles alternates proceeding around the ring.
The magnet elements are typically made of hard ferrite permanent magnet material. The magnets may be isotropic or anisotropic. In anisotropic, the magnet elements are formed with their magnetic domains aligned across the thickness of the magnet so as to be aligned radially generally as shown by arrow “A” in FIG. 1 of the present application. Magnetisation of the rotor follows this pattern to create radial magnetic field lines through the thickness of the magnet, represented by the magnetic flux lines or paths in FIG. 1. This results in a pattern of poles on the outside face of the magnets (adjacent the backing steel) that is the inverse of the pattern of poles on the inside of the face of the magnets (facing radial inwards).
In the case of radial magnetisation, the portion of each magnet close to the interface between magnets is known to provide little benefit in terms of the flux coupled from the rotor into the stator and can typically be removed with little loss in torque production.
Halbach arrays have been created to at least partially alleviate this problem. One example of a Halbach array is an arrangement of magnets with their respective directions of magnetisation oriented as shown in FIG. 2a of the present application. As shown in FIG. 2b of the present application, a total resulting magnetic flux field is produced that reduces the magnetic flux that couples out the back face of the magnetic ring. Isotropic or anisotropic magnetic sections can be used in such an array. Anisotropic sections have magnetic domains aligned in one direction, whereas isotropic sections have magnetic domains arranged randomly. FIG. 2c shows a portion of an “ideal” Halbach array resulting magnetic flux field where a large or infinite number of magnetic elements are formed into a Halbach array.
It has been proposed that a single piece isotropic ring can be magnetised to produce a Halbach array “style” magnetic field. The sections of single piece ring are magnetised using an external magnetic field. Performance of the isotropic ring will be limited relative to radially magnetised anisotropic magnets due to the reduced magnetic strength of the isotropic magnets.